For industrial purposes, it is frequently necessary to rapidly combine streams of liquids and solids to form solutions on a continuous basis. The problems encountered in forming uniform solutions by mixing powdered or granulated solids with liquids have been researched extensively. However, dissolving hard-to-wet and/or hard to dissolve materials, such as certain polymers, is not an easy task, as is now explained.
Many water soluble polymers, such as polyvinyl alcohol (PVA), cellulose derivatives, such as hydroxyethyl cellulose, carboxymethyl cellulose and the like, are soluble in water but are nevertheless very difficult to dissolve. The polymer particles adhere strongly to one another on wetting and tend to form lumps. In most traditional mixing devices, such lumps become wetted before the particles disperse into individual particles. The wetted surface of a lump becomes an impermeable film that hinders break up of the lump, and the lumps are carried through the mixer with the powder inside remaining substantially dry and unmixed with the liquid.
In the prior art, preparing solutions of hard-to-wet and/or hard to dissolve polymer powders is done as a batch process. For example, ambient temperature water is fed into a blend tank, and the water is agitated to form a vortex. The powder is then dispersed in the ambient water by gradually adding it to the vortex. The agitated mixture of powder and water is heated using, to a specific cure temperature. The mixture is held and agitated at the cure temperature for the time required to dissolve the powder. PVA, for example, is first formed into a slurry in ambient temperature water and then usually heated to a temperature of at least 90° C. Under these conditions, the complete dissolution of the slurry typically takes 30 to 60 minutes and yields no greater than a 10% solution. Hydroxyethyl cellulose is another hard-to-wet powder which, although curing at ambient temperature, usually requires at least two hours to form a complete solution.
There are many disadvantages with the prior art method. It is inefficient, costly, capital intensive and time-consuming. The powder is added to water at ambient temperature with high agitation to disperse the powder. If the water is at an elevated temperature, the powder clumps more readily. Once the powder is relatively well dispersed, the mixture must be heated and held at the higher temperature in order to dissolve the polymer. The mixing, heating, and curing cycle is slow. In addition, the space required for the blend tank may present a problem in installing a polymer solution system in an existing plant. Also problematic is that undissolved powder clumps can remain in the solution and result in inconsistent solution properties. Solution aeration due to the high speed agitation required for polymer dispersion and excessive foaming due to the heat-curing requirement are additional problems. The fact that the prior art must work with batches is another disadvantage; it is logistically difficult and costly to work with large amounts/containers of raw material and large, intermediate storage inventories.
Other methods have been proposed to tackle these problems. For example, processes have been described that use two mixing vessels. In the first mixing vessel, a high-molecular weight polymer is combined with a solvent and agitated to form a slurry. More intensive mixing and agitating occurs in the second vessel to convert the slurry into a solution. Another process attempts to use a jet liquid spray to break up lumps of the polymer powder.